This invention relates to improvements in oil-cooled compressors.
In conventional oil-cooled compressors, e.g. screw compressors, a gas, usually air, is compressed and is sucked into the compressor together with oil; the oil acts to cool and lubricate the main body of the compressor. The compressed gas discharged from the main body of the compressor is separated from the oil in an oil tank or a separator and is introduced into a pressure tank, while the oil separated from the compressed gas is cooled and, thereafter, introduced into the main body of the compressor.
When the pressure in the pressure tank elevates to a predetermined maximum pressure, a pressure switch used for detecting the pressure opens so as to stop a motor which is driving the main body of the compressor. When the pressure in the pressure tank decreases to a predetermined minimum pressure by consuming the pressurized gas in the pressure tank, the pressure switch closes so as to drive the motor.
The pressurized gas discharged out of the main body of the compressor has a high temperature and a high humidity and, when the temperature of the oil tank itself and the oil contained therein is at a relatively low temperature such as in an early stage of the operation, the gas is cooled in the oil tank, thereby generating condensation which causes rust in the oil tank and in the main body of the compressor and also deteriorates the characteristics of the oil. Namely, the temperature of the oil in the oil-cooled compressor should be low from in the viewpoint of cooling; however, it is necessary for the oil temperature to be high for preventing the generation of condensation, (the temperature being determined by the dew point of the compressed gas, and is about 50.degree. C. or more when the pressure of the compressed air is 8-9 kg/cm.sup.2). Therefore, there have been provided a temperature controlled valve located between the oil tank and an oil cooler, and a by-passing pipe connecting the temperature controlled valve with the suction port of the main body of the compressor by-passing the oil cooler, whereby the amount of oil flowing through the oil cooler is controlled in accordance with the temperature of the oil.
However, in the aforesaid prior art device, the temperature of the oil does not quickly elevate from a low temperature such as a room temperature to a desired minimum operating temperature such as the temperature corresponding to the dew point of the compressed gas. Particularly, when the operation of the motor is controlled by the pressure switch, the duration of the operation of the motor is relatively short; thus, the temperature of the oil does not elevate to the desired minimum operating temperature in a short period of time. Particularly, when the pressure in the reservoir tank is near to the maximum pressure, the duration of the operation of the motor decreases accordingly, and the oil temperature will not rise to the desired operating temperature.